High speed optical scanning systems such as precision plotters, printers and the like are well known in the art. These devices are direct imaging systems and are used to fabricate printed circuit boards (PCB) and printing plates by raster scanning an exposure beam across a film emulsion which is further processed into a printed circuit board phototool or a printing plate artwork, respectively. A typical system as marketed by the Gerber Scientific Instrument Company, the assignee of the present application, consists of a magnetic tape drive, hard disk, computer interactive graphics terminal, image processor, and optical table having a scanner. The system may also include such optics, media carriage and electronics as is necessary to directly transfer computer aided design (CAD) data to a printed circuit board phototool or transfer fonts, graphics and half-toned images into a printing plate artwork.
In operation, the direct imaging system is configured to receive on a write platen a planar substrate of film which has an optically sensitive media upon its surface. The computer provides signals to an exposure beam generator that modulates the intensity of a scanned optical exposure beam, usually provided by a laser, to expose selected portions of the substrate. Typically, a reference beam is scanned across a reference mask and a detector simultaneously with the exposing beam for providing a scanned timing signal to accurately indicate the position of the exposure (or write beam) on the substrate. A flat field scanning system is sometimes employed to focus the beams and to accomplish the simultaneous scanning of the beams across a reference mask and the substrate, respectively. Precision air bearings are often used to guide the write platen as the substrate is scanned or "imaged".
In known laser raster scanning systems where the optical deflector is a multi-faceted mirror, the mirror is rotated at a fixed, selected speed such that the exposure beam is advanced at a constant rate. However, it is possible with these known systems to have the computation rate of pixel signals or "pixels" from the computer fall behind the imaging rate of the exposure beam. When this occurs, only a portion of the scan line will be exposed.
It would be advantageous to have a laser raster scanning system which automatically compensates for unacceptable differences between the rate at which the computer generates signals to modulate the exposure beam and the system scan or imaging rate. The present invention is drawn towards such a system.